System and method for exchanging tape cartridges in an automated tape cartridge library system

ABSTRACT

A system and method for exchanging tape cartridges between two automated tape cartridge libraries. A carriage assembly carries cartridges back and forth between the libraries. The carriage assembly includes a cartridge transport cell, has a vertical axis, and cooperates with pass through ports in the libraries for providing access to tape cartridge storage cells in the libraries. A guide structure extends between the libraries for supporting the carriage assembly. The guide structure defines a linear path and has first and second sides substantially parallel to the linear path which define an envelope. The carriage assembly is driven along the guide structure, and automatically rotates about the substantially vertical axis. The cartridge transport cell is accessible by robotic devices in the libraries when the carriage assembly is aligned with the pass through ports. The carriage assembly rotates substantially within the envelope as it is driven along the guide structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisionalapplication Ser. No. 60/302,248 filed Jun. 29, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the exchange of tapecartridges between tape cartridge libraries and, more particularly, to asystem and method for exchanging tape cartridges between automated tapecartridge libraries linked in a library system.

[0004] 2. Background

[0005] Current automated libraries for tape cartridges typically includearrays of multiple storage cells housing the tape cartridges, as well asmultiple media drives. Multiple automated robotic devices may be used tomove tape cartridges between the various storage cells and media driveswithin a library. The use of multiple robotic devices raises variousproblems concerning the distribution of power to such robotic devices.In prior art automated tape cartridge libraries, the movement of therobotic devices is restricted by wire cable connections used forproviding such power. Such cabling can prevent the robotic devices fromcrossing paths, or from continuous movement in one direction around thelibrary without the necessity of ultimately reversing direction.

[0006] Such problems can be overcome through the use of powerdistribution strips or rails. A robotic device traveling over a givenroute may used fixed conductive strips or rails to supply power to therobotic device, which itself is provided with brushes or wheels thatcontact the conductive strips or rails in order to conduct power to therobotic device. The integration of such conductive strips or rails intothe automated tape cartridge library, in conjunction with brush or wheelcontacts provided on the robotic devices, allows for greater freedom ofmovement of the robotic devices. Moreover, such power distributionsystems also provide for modular and extensible power distribution torobotic devices as library configurations change, or as libraries areconnected in a modular fashion to form library systems.

[0007] In that regard, a number of relatively large automated tapecartridge libraries are known having, for example, between 1000 and 2500cartridge storage cells. Relatively smaller automated tape cartridgelibraries, having for example on the order of 700 cartridge storagecells, are also known. For some users, as storage requirements increase,a smaller automated tape cartridge library may have to be replaced witha larger automated library, often at considerable expense. As a result,there exists a need to allow such smaller automated libraries to belinked together to create larger library systems having, for example, onthe order of 1400 cartridge storage cells.

[0008] It is known to link the above described larger automatedlibraries using a variety of pass-through ports and cartridge exchangemechanisms. For example, to ensure maximum access to data stored in thetape cartridges, multiple paths are created between larger automatedlibraries that employ cam or screw driven cartridge exchange devices. Assuch devices are located entirely or substantially within a library,little floor space in a data center is compromised by the device.However, the use of multiple paths and interior cartridge exchangedevices result in the loss of a large number of cartridge storage cellsin each library.

[0009] Alternatively, multiple larger automated libraries can be linkedlinearly to improve access to data. In such configurations, gravitydriven cartridge exchange devices are employed between linked libraries.In operation, a tape cartridge is delivered to the cartridge exchangedevice by a robotic device in one of the libraries. The weight of thecartridge causes the exchange device to rotate and deliver the cartridgeto the linked library, where it can be retrieved by another roboticdevice in that library. This configuration does not result in the lossof large number of storage cells in each library, and only minimallyaffects floor space. However, only a single cartridge may be passed at atime and, because the exchange device is gravity driven, another pathmust be provided to exchange cartridges in the opposite directionbetween the linked libraries. Moreover, more complex host software whichsees the linked libraries as a single library, such as AutomatedCartridge System Library Server (ACSLS), may be required to control themultiple linked libraries.

[0010] The pass-through ports and cartridge exchange mechanisms used inlinking the relatively larger automated libraries, however, are notsuitable for use in linking the relatively smaller automated librariesin that they do not meet the form factors (e.g., orientation, floorspace) required by users needing 1400 cartridge storage cells. Moreover,pass-through ports and cartridge exchange devices that result in theloss of large numbers of cartridge storage cells are unacceptablebecause such a loss represents a significant percentage of storage spacein such smaller libraries. Still further, the development of new tapecartridges having differing weights raises concerns as to the ability ofgravity driven cartridge exchange devices to reliably transfer all typesof cartridges.

[0011] Thus, there exist a need for an improved cartridge exchangemechanism for use in linked automated libraries. Such an improvedmechanism would preferably minimize the number of storage cells that maybe lost as a result of linking automated libraries, as well aspreferably minimize the amount of space required for linking suchlibraries. Such an improved mechanism would also preferably be capableof reliably transferring a wide variety of cartridge types.

SUMMARY OF THE INVENTION

[0012] Accordingly, the present invention provides an improved systemand method for exchanging tape cartridges between first and second tapecartridge libraries.

[0013] According to the present invention, then, in an automated tapecartridge library system having a first library and a second library,each of the first and second libraries having a plurality of storagecells for housing tape cartridges and a robotic device for moving thetape cartridges in the library, a system is provided for exchanging tapecartridges between the first and second libraries. The system comprisesa carriage assembly for carrying tape cartridges from the first libraryto the second library and from the second library to the first library,the carriage assembly comprising at least one cartridge transport cell,wherein the carriage assembly has a substantially vertical axis and isadapted to cooperate with a first pass through port in the first libraryfor providing access to the plurality of storage cells in the firstlibrary and a second pass through port in the second library forproviding access to the plurality of storage cells in the secondlibrary. The system further comprises a guide structure extendingbetween the first and second libraries for supporting the carriageassembly, the guide structure defining a linear path and having firstand second sides substantially parallel to the linear path, the firstand second sides defining an envelope. The system still furthercomprises means for driving the carriage assembly along the guidestructure between the first and second pass through ports, and means forrotating the carriage assembly about the substantially vertical axis,wherein the at least one cartridge transport cell is accessible by therobotic device of the first library when the carriage assembly isaligned with the first pass through port, the rotating meansautomatically rotates the carriage assembly substantially within theenvelope as the driving means drives the carriage assembly along theguide structure between the first and second libraries, and the at leastone cartridge transport cell is accessible by the robotic device of thesecond library when the carriage assembly is aligned with the secondpass through port.

[0014] Also according to the present invention, in an automated tapecartridge library system having a first library and a second library,each of the first and second libraries having a plurality of storagecells for housing tape cartridges and a robotic device for moving thetape cartridges in the library, a method is provided for exchanging tapecartridges between the first and second libraries. The method comprisesproviding a carriage assembly for carrying tape cartridges from thefirst library to the second library and from the second library to thefirst library, the carriage assembly comprising at least one cartridgetransport cell, wherein the carriage assembly has a substantiallyvertical axis and is adapted to cooperate with a first pass through portin the first library for providing access to the plurality of storagecells in the first library and a second pass through port in the secondlibrary for providing access to the plurality of storage cells in thesecond library. The method further comprises providing a guide structureextending between the first and second libraries for supporting thecarriage assembly, the guide structure defining a linear path and havingfirst and second sides substantially parallel to the linear path, thefirst and second sides defining an envelope. The method still furthercomprises providing means for driving the carriage assembly along theguide structure between the first and second pass through ports, andproviding means for rotating the carriage assembly about thesubstantially vertical axis, wherein the at least one cartridgetransport cell is accessible by the robotic device of the first librarywhen the carriage assembly is aligned with the first pass through port,the rotating means automatically rotates the carriage assemblysubstantially within the envelope as the driving means drives thecarriage assembly along the guide structure between the first and secondlibraries, and the at least one cartridge transport cell is accessibleby the robotic device of the second library when the carriage assemblyis aligned with the second pass through port.

[0015] The above features, and other features and advantages of thepresent invention are readily apparent from the following detaileddescriptions thereof when taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a perspective view of a robotic device for use in anautomated tape cartridge library having brush and strip powerdistribution;

[0017]FIGS. 2a and 2 b are partial cross-sectional views of a roboticdevice for use in an automated tape cartridge library having brush andstrip power distribution;

[0018] FIGS. 3-6 are perspective, side and cross-sectional views,respectively, of a robotic device for use in an automated tape cartridgelibrary having brush and wheel power distribution;

[0019]FIG. 7 is a perspective view of a cartridge exchange device foruse in an automated tape cartridge library system according to thepresent invention;

[0020]FIG. 8 is an exploded view of a cartridge exchange device for usein an automated tape cartridge library system according to the presentinvention;

[0021]FIG. 9 is a top view of a cartridge exchange device installedbetween two automated tape cartridge libraries according to the presentinvention;

[0022]FIG. 10 is a perspective view of a cartridge exchange deviceinstalled between two automated tape cartridge libraries according tothe present invention;

[0023]FIG. 11 is an exploded view of a cartridge exchange deviceinstalled between two automated tape cartridge libraries according tothe present invention;

[0024]FIG. 12 is a partial perspective view of an array of multiplecartridge storage cells in the interior of an automated tape cartridgelibrary;

[0025]FIG. 13 is a partial perspective view of an array of multiplecartridge storage cells in the interior of an automated tape cartridgelibrary having an installed cartridge exchange device according to thepresent invention; and

[0026]FIG. 14 is a simplified, exemplary flowchart depicting the methodof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0027] With reference to the Figures, the preferred embodiments of thepresent invention will now be described in greater detail. The presentapplication incorporates by reference herein commonly owned U.S. patentapplication Ser. Nos. ______ (Attorney Docket No. 2001-047-TAP), ______(Attorney Docket No. 2001-048-TAP), (Attorney Docket No. 2001-049-TAP),______ (Attorney Docket No. 2001-050-TAP;), and ______ (Attorney DocketNo. 2001-100-TAP), all filed on the same date as the presentapplication.

[0028] As previously noted, current automated libraries for tapecartridges typically include arrays of multiple storage cells housingthe tape cartridges, as well as multiple media drives. Multipleautomated robotic devices may be used to move tape cartridges betweenthe various storage cells and media drives within a library. As alsonoted previously, the use of multiple robotic devices raises variousproblems concerning the distribution of power thereto. In prior artautomated tape cartridge libraries, the movement of the robotic devicesis restricted by wire cable connections used for providing such power.Such cabling can prevent the robotic devices from crossing paths, orfrom continuous movement in one direction around the library without thenecessity of ultimately reversing direction.

[0029] Such problems can be overcome through the use of powerdistribution strips or rails. A robotic device traveling over a givenroute may used fixed conductive strips or rails to supply power to therobotic device, which itself is provided with brushes or wheels thatcontact the conductive strips or rails in order to conduct power to therobotic device. The integration of such power distribution strips orrails into the automated tape cartridge library, in conjunction withbrush or wheel contacts provided on the robotic devices, allows forgreater freedom of movement of the robotic devices, as well as formodular and extensible power distribution to robotic devices as libraryconfigurations change, or as libraries are connected in a modularfashion to form library systems.

[0030] In that regard, FIGS. 1 and 2a-b show perspective andcross-sectional views, respectively, of a robotic device for use in anautomated tape cartridge library having brush and strip powerdistribution. As seen therein, a moveable robotic device (20), which maybe referred to as a “handbot” or “picker,” is supported by a guidestructure or rail (2) preferably having an integrated power strip (1).Guide rail (2) and/or power strip (1) may also be referred to as atrack. Power strip (1) preferably comprises back-to-back conductivesurfaces (1A, 1B), preferably copper, separated by a dielectric(preferably FR4) in a sandwich-like configuration. Power strip (1) maybe a printed circuit board wherein copper conductors are laminated,glued or etched onto a substrate material. Alternatively, power strip(1) may comprise copper foil tape glued or laminated onto plasticmaterial, or copper inserts molded into a moldable plastic material. Anyother methods of construction or configurations known to those ofordinary skill may also be used.

[0031] Robotic device (20) includes brush contacts (6) for providingpower to robotic device (20). In that regard, the back-to-backconductive surfaces (1A, 1B) of power strip (1) are oppositely charged.An upper brush (6A) in contact with one conductive surface (1A), inconjunction with a corresponding lower brush (6B) in contact with theopposite conductive surface (1B) thereby supply power to the roboticdevice (20). Brushes (6) are contained in housing assembly (7) and, toensure that contact between brushes (6) and power strip (1) ismaintained, brushes (6) are spring loaded (8). Multiple or redundantpairs of such upper and lower brushes (6) are preferably provided, andpreferably spring loaded (8) independently, to improve robustness andreliability in the event of a brush failure, momentary loss of contactat one or more brushes due to any track irregularities, including seamsor joints therein, or voltage irregularities between adjacent powerstrips (1). Moreover, brushes (6) preferably have a circularcross-section, such as is provided by a cylindrical shaped brush (6), asthese are better able to traverse a joint or seam (38) in the powerstrip (1), which may more readily impede or catch a square shaped brush.

[0032] Power supplied to robotic device (20) through power strip (1) andbrushes (6) powers a motor (not shown) in robotic device (20), which inturn drives a belt and gear system (22). Guide rails (2) includes teeth(24) which cooperate with belt and gear system (22) to permit roboticdevice (20) to move back and forth along guide rails (2) via guidewheels (26). As seen in FIG. 1, robotic device (20) may thereby gainaccess to tape cartridges stored in library cells (28) located adjacentguide rail (2).

[0033] Referring next to FIGS. 3 through 6a and 6 b, variousperspective, side and cross-sectional views of a robotic device for usein an automated tape cartridge library having brush and wheel powerdistribution are shown. As seen therein, in this alternative embodiment,robotic device (20) is supported by a guide rail (2), which is providedwith a pair of oppositely charged power conductors (3), preferably inthe form of copper rails. Power rails (3) supply power to robotic device(20) through power transmission carriage assembly (4). Power supplied torobotic device (20) via power rails (3) and power transmission carriage(4) powers a motor (not shown), which in turn drives belt and gearmechanism (22) to permit robotic device (20) to move back and forthalong guide rail (2) via guide wheels (26).

[0034] Power transmission carriage (4) includes multiple cooperatingpairs of conduction wheels (5) (preferably copper), the individualmembers of a cooperating pair provided in contact, respectively, withoppositely charged conductor rails (3). Conductive brushes (10) areprovided to contact conduction wheels (5) and are spring loaded (11),preferably independently, to maintain such contact. To maintain contactbetween conduction wheels (5) and conductor rails (3), powertransmission carriage (4) also includes vertical pre-load spring (6).Power transmission carriage (4) still further includes gimbal arm (7)with pivot shaft (8) and pivot screw (9) for carriage compliance. Onceagain, multiple or redundant conduction wheel (5) and conductive brush(10) pairs are preferably provided, and preferably spring loaded (11)independently, to improve robustness and reliability in the event of abrush failure, momentary loss of contact at one or more wheels due toany track irregularities, including seams or joints therein, or voltageirregularities between adjacent power rails (3). In that same regard,while a single vertical pre-load spring (6) is shown, each conductionwheel (5) could also be independently spring loaded to maintain contactwith conductor rails (3), thereby allowing for better negotiation of anytrack irregularities or imperfections, including joints or seams.

[0035] As previously described, a number of relatively large automatedtape cartridge libraries are known having, for example, between 1000 and2500 cartridge storage cells. Relatively smaller automated tapecartridge libraries, having for example on the order of 700 cartridgestorage cells, are also known. For some users, as storage requirementsincrease, a smaller automated tape cartridge library may have to bereplaced with a larger automated library, often at considerable expense.As a result, there exists a need to allow such smaller automatedlibraries to be linked together to create larger library systems having,for example, on the order of 1400 cartridge storage cells.

[0036] In that regard, it is known to link the above described largerautomated libraries using a variety of pass-through ports and cartridgeexchange mechanisms. For example, to ensure maximum access to datastored in the tape cartridges, multiple paths are created between largerautomated libraries that employ cam or screw driven cartridge exchangedevices. As such devices are located entirely or substantially within alibrary, little floor space in a data center is compromised by thedevice. However, the use of multiple paths and interior cartridgeexchange devices result in the loss of a large number of cartridgestorage cells (e.g., 24) in each library.

[0037] Alternatively, multiple larger automated libraries can be linkedlinearly to improve access to data. In such configurations, gravitydriven cartridge exchange devices are employed between linked libraries.In operation, a tape cartridge is delivered to the cartridge exchangedevice by a robotic device in one of the libraries. The weight of thecartridge causes the exchange device to rotate and deliver the cartridgeto the linked library, where it can be retrieved by another roboticdevice in that library. This configuration does not result in the lossof large number of storage cells (e.g., 4) in each library, and onlyminimally affects floor space. However, only a single cartridge may bepassed at a time and, because the exchange device is gravity driven,another path must be provided to exchange cartridges in the oppositedirection between the linked libraries. Moreover, more complex hostsoftware which sees the linked libraries as a single library, such asAutomated Cartridge System Library Server (ACSLS), may be required tocontrol the multiple linked libraries.

[0038] The pass-through ports and cartridge exchange mechanisms used inlinking the relatively larger automated libraries, however, are notsuitable for use in linking the relatively smaller automated librariesin that they do not meet the form factors (e.g., orientation, floorspace) required by users needing 1400 cartridge storage cells. Moreover,pass-through ports and cartridge exchange devices that result in theloss of large numbers of cartridge storage cells are unacceptablebecause such a loss represents a significant percentage of storage spacein such smaller libraries. Still further, the development of new tapecartridges having differing weights raises concerns as to the ability ofgravity driven cartridge exchange devices to reliably transfer all typesof cartridges.

[0039] As a result, as previously discussed, there exists a need for animproved cartridge exchange mechanism for linking automated libraries.Such an improved mechanism would preferably minimize the number ofstorage cells that may be lost as a result of linking automatedlibraries, as well as preferably minimize the amount of space requiredfor linking such libraries. Such an improved mechanism would alsopreferably be capable of reliably transferring a wide variety ofcartridge types.

[0040] Referring now to FIGS. 7 and 8, perspective and exploded views ofthe cartridge exchange device for use in an automated tape cartridgelibrary system according to the present invention is shown. Such adevice is preferably for use in exchanging tape cartridges between twolinked automated libraries having, for example, 700 cartridge storagecells each. In that regard, referring now to FIGS. 9-11, top,perspective and exploded views of the cartridge transport deviceinstalled between two such automated tape cartridge libraries accordingto the present invention are shown. As seen therein, and with continuingreference to FIGS. 7 and 8, cartridge exchange device (100) preferablycomprises a carriage assembly (102) for transporting tape cartridgesbetween automated libraries (104, 105). Carriage assembly (102) ispreferably provided with capacity for two tape cartridges, having twotape transport cells (106).

[0041] In contrast to prior art cam or screw driven cartridge exchangedevices which require extremely tight tolerances and expensivecomponents, and which must be installed and set-up to meet specificdistance requirements between libraries, device (100) is preferablymotor and belt driven (108) and self-calibrated to ensure robust design.Motor and belt (108) drive carriage assembly (102) back and forthbetween libraries (104, 105) along guide structure (110). In operation,carriage assembly (102) is originally located at one end of guidestructure (110) adjacent a pass-through port or window (not shown) inone library (104), which may be referred to as a “home” position. One ortwo tape cartridges to be transported from library (104) to library(105) are delivered and inserted into transport cells (106) by a roboticdevice (not shown) within library (104) through the window (not shown).As carriage assembly (102) is driven along guide structure (110) towardlibrary (105) to complete the exchange, rack and pinion gears (112, 114)operate to cause carriage assembly (102) to rotate approximately 180° sothat transport cells (106) face a pass-through port or window (notshown) in library (105). Upon arrival of carriage assembly (102) atlibrary (105), which may be referred to as an “away” position, the oneor more tape cartridges in transport cells (106) are retrieved by one ormore robotic devices (not shown) within library (105) through the window(not shown). The use of such rack and pinion gears (112, 114) permitscarriage assembly (102) to rotate in a relatively small space, allowingdevice (100) to operate within the form factors, particularly floorspace, required by users of such library systems.

[0042]FIG. 12 depicts a partial perspective view of an array of multiplecartridge storage cells (120) in the interior of an automated tapecartridge library. FIG. 13 depicts a partial perspective view of anarray of multiple cartridge storage cells (120) in the interior of anautomated tape cartridge library (104, 105) having an installedcartridge exchange device (100) according to the present inventions. Asseen from those figures, as well as from FIGS. 9-11, the lineartransport path of device (100) between libraries (104, 105) results inminimal loss of cartridge storage cells (120). As shown in FIG. 13, onlysix such storage cells (120) are lost per library (104, 105), althoughdevice (100) occupies the space of only three such storage cells (120).FIG. 24 also shows the narrow pass-through port or window (122) requiredfor operation of device (100), again limiting the loss of storage cells(120) and increasing tape cartridge density within libraries (104, 105).Moreover, as device (100) is not gravity driven, it can reliablytransport all types of tape cartridges and need not be fine tuned inlight of the development of new cartridges.

[0043] Referring next to FIG. 14, a simplified, exemplary flowchartdepicting the method of the present invention is shown, denotedgenerally by reference numeral 150. The method (150) is for exchangingtape cartridges between first and second libraries and is provided foruse in an automated tape cartridge library system having a first libraryand a second library, each of the first and second libraries having aplurality of storage cells for housing tape cartridges and a roboticdevice for moving the tape cartridges in the library. As seen in FIG.14, the method (150) comprises providing a carriage assembly forcarrying tape cartridges from the first library to the second libraryand from the second library to the first library, the carriage assemblycomprising at least one cartridge transport cell, wherein the carriageassembly has a substantially vertical axis and is adapted to cooperatewith a first pass through port in the first library for providing accessto the plurality of storage cells in the first library and a second passthrough port in the second library for providing access to the pluralityof storage cells in the second library.

[0044] The method (150) further comprises providing a guide structureextending between the first and second libraries for supporting thecarriage assembly, the guide structure defining a linear path and havingfirst and second sides substantially parallel to the linear path, thefirst and second sides defining an envelope. The method (150) stillfurther comprises providing means for driving the carriage assemblyalong the guide structure between the first and second pass throughports, and providing means for rotating the carriage assembly about thesubstantially vertical axis, wherein the at least one cartridgetransport cell is accessible by the robotic device of the first librarywhen the carriage assembly is aligned with the first pass through port,the rotating means automatically rotates the carriage assemblysubstantially within the envelope as the driving means drives thecarriage assembly along the guide structure between the first and secondlibraries, and the at least one cartridge transport cell is accessibleby the robotic device of the second library when the carriage assemblyis aligned with the second pass through port.

[0045] It should be noted that the simplified flowchart depicted in FIG.14 is exemplary of the method of the present invention. In that regard,the steps of such method may be executed in sequences other than thoseshown in FIG. 14, including the execution of one or more stepssimultaneously.

[0046] Thus it is apparent that the present invention provides animproved cartridge exchange mechanism for linking automated libraries.In that regard, the cartridge exchange device of the present inventionminimizes the number of storage cells that may be lost as a result oflinking automated libraries, as well as minimizes the amount of spacerequired for linking such libraries. The cartridge exchange device ofthe present invention is further capable of reliably transferring a widevariety of cartridge types.

[0047] While embodiments of the invention have been illustrated anddescribed, it is not intended that these embodiments illustrate anddescribe all possible forms of the invention. Rather, the words used inthe specification are words of description rather than limitation, andit is understood that various changes may be made without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. In an automated tape cartridge library systemhaving a first library and a second library, each of the first andsecond libraries having a plurality of storage cells for housing tapecartridges and a robotic device for moving the tape cartridges in thelibrary, a system for exchanging tape cartridges between the first andsecond libraries, the system comprising: a carriage assembly forcarrying tape cartridges from the first library to the second libraryand from the second library to the first library, the carriage assemblycomprising at least one cartridge transport cell, wherein the carriageassembly has a substantially vertical axis and is adapted to cooperatewith a first pass through port in the first library for providing accessto the plurality of storage cells in the first library and a second passthrough port in the second library for providing access to the pluralityof storage cells in the second library; a guide structure extendingbetween the first and second libraries for supporting the carriageassembly, the guide structure defining a linear path and having firstand second sides substantially parallel to the linear path, the firstand second sides defining an envelope; means for driving the carriageassembly along the guide structure between the first and second passthrough ports; and means for rotating the carriage assembly about thesubstantially vertical axis, wherein the at least one cartridgetransport cell is accessible by the robotic device of the first librarywhen the carriage assembly is aligned with the first pass through port,the rotating means automatically rotates the carriage assemblysubstantially within the envelope as the driving means drives thecarriage assembly along the guide structure between the first and secondlibraries, and the at least one cartridge transport cell is accessibleby the robotic device of the second library when the carriage assemblyis aligned with the second pass through port.
 2. The system of claim 1wherein the means for rotating the carriage assembly comprises a linearrack fixed to the guide structure and a pinion gear fixed to thecarriage assembly, the rack and pinion gear cooperating to automaticallyrotate the carriage assembly about the substantially vertical axisbetween a first position for alignment with the first pass through portand a second position for alignment with the second pass through port.3. The system of claim 2 wherein the rack and pinion gear rotate thecarriage assembly approximately 180° about the vertical axis.
 4. Thesystem of claim 1 wherein the means for driving the carriage assemblyalong the guide structure comprises a motor and belt assembly.
 5. Thesystem of claim 1 wherein the means for driving the carriage assemblyalong the guide structure comprises a motor and screw assembly.
 6. Thesystem of claim 1 wherein the carriage assembly is suspended from theguide structure.
 7. The system of claim 1 wherein the first and secondlibraries each comprise approximately 700 tape cartridge storage cells.8. The system of claim 1 wherein the automated tape cartridge librarysystem further comprises a controller for controlling the roboticdevices of the first and second libraries, and the cartridge exchangesystem is adapted to be controlled by the controller.
 9. The system ofclaim 1 wherein the cartridge exchange system is located substantiallyoutside both the first and the second libraries to reduce a number oftape cartridge storage cells in each of the first and second librarieslost as a result of linking the first and second libraries in theautomated library system.
 10. The system of claim 9 wherein the at leastone cartridge transport cell comprises two cartridge transport cells sothat the first and second pass through ports are provided with asufficiently small area to reduce the number of tape cartridge storagecells in each of the first and second libraries lost as a result of thelinking the first and second libraries in the automated library system.11. In an automated tape cartridge library system having a first libraryand a second library, each of the first and second libraries having aplurality of storage cells for housing tape cartridges and a roboticdevice for moving the tape cartridges in the library, a method forexchanging tape cartridges between the first and second libraries, themethod comprising: providing a carriage assembly for carrying tapecartridges from the first library to the second library and from thesecond library to the first library, the carriage assembly comprising atleast one cartridge transport cell, wherein the carriage assembly has asubstantially vertical axis and is adapted to cooperate with a firstpass through port in the first library for providing access to theplurality of storage cells in the first library and a second passthrough port in the second library for providing access to the pluralityof storage cells in the second library; providing a guide structureextending between the first and second libraries for supporting thecarriage assembly, the guide structure defining a linear path and havingfirst and second sides substantially parallel to the linear path, thefirst and second sides defining an envelope; providing means for drivingthe carriage assembly along the guide structure between the first andsecond pass through ports; and providing means for rotating the carriageassembly about the substantially vertical axis, wherein the at least onecartridge transport cell is accessible by the robotic device of thefirst library when the carriage assembly is aligned with the first passthrough port, the rotating means automatically rotates the carriageassembly substantially within the envelope as the driving means drivesthe carriage assembly along the guide structure between the first andsecond libraries, and the at least one cartridge transport cell isaccessible by the robotic device of the second library when the carriageassembly is aligned with the second pass through port.
 12. The method ofclaim 11 wherein the means for rotating the carriage assembly comprisesa linear rack fixed to the guide structure and a pinion gear fixed tothe carriage assembly, the rack and pinion gear cooperating toautomatically rotate the carriage assembly about the substantiallyvertical axis between a first position for alignment with the first passthrough port and a second position for alignment with the second passthrough port.
 13. The method of claim 12 wherein the rack and piniongear rotate the carriage assembly approximately 180° about the verticalaxis.
 14. The method of claim 11 wherein the means for driving thecarriage assembly along the guide structure comprises a motor and beltassembly.
 15. The method of claim 11 wherein the means for driving thecarriage assembly along the guide structure comprises a motor and screwassembly.
 16. The method of claim 11 wherein the carriage assembly issuspended from the guide structure.
 17. The method of claim 11 whereinthe first and second libraries each comprise approximately 700 tapecartridge storage cells.
 18. The method of claim 11 wherein theautomated tape cartridge library system further comprises a controllerfor controlling the robotic devices of the first and second libraries,and the cartridge exchange system is adapted to be controlled by thecontroller.
 19. The method of claim 11 wherein the cartridge exchangesystem is located substantially outside both the first and the secondlibraries to reduce a number of tape cartridge storage cells in each ofthe first and second libraries lost as a result of linking the first andsecond libraries in the automated library system.
 20. The method ofclaim 19 wherein the at least one cartridge transport cell comprises twocartridge transport cells so that the first and second pass throughports are provided with a sufficiently small area to reduce the numberof tape cartridge storage cells in each of the first and secondlibraries lost as a result of the linking the first and second librariesin the automated library system.